In a compound semiconductor device, particularly in a high-power high-frequency device which includes a high electron mobility transistor (HEMT) of nitride such as GaN, a high electric field is apt to be applied to a periphery of a gate electrode. If a part of electrons accelerated by the electric field is transited and trapped by traps, the part of electron broadens a depletion layer and lowers a current. It is known that traps exist in an interface between a passivation film such as a silicon nitride film and a compound semiconductor layer, and inside and on a surface of a passivation film. Such a phenomenon is peculiar to a nitride semiconductor device and is called a current collapse. The current collapse lowers output characteristics of an amplifier such as output and efficiency. In a case where a structure of a gate electrode is of an overhang type, a depletion layer is hard to be broadened due to existence of traps during the off-time, and thus a gate leakage current is apt to increase.
[Patent Literature 1] Japanese Laid-open Patent Publication No. 2011-192719.